heat pump glossary

HVAC and air source heat pump glossary: key terms to know

If you’re planning on installing heat pumps any time soon – whether you’re replacing your current heating and cooling system or starting from scratch – there can be a lot of jargon to sift through as you navigate the market. We’ve put together a glossary of some of the common terms you’re likely to come across as you research your purchase. 

View common terms by letter: ABCDEGHKLMOPRSTVW


Air changes per hour (ACH)

Air changes per hour refers to the number of times during an hour-long period of time that air is supplied to or removed from a room through both mechanical ventilation (your heating and cooling system) and natural ventilation. 

Air conditioner (AC)

An air conditioner is an appliance or system that removes heat and humidity from a room or entire building. Air conditioners can be installed in your window or wall, and deliver cool air to a room or area of your home without ductwork. Central air conditioning delivers cool air to all of the rooms in your home from one large, central unit via fans and air ducts.

Air handler

The indoor component of a heating and cooling system that circulates air throughout a building.

Air source heat pump (ASHP)

An air source heat pump extracts heat from the air and transfers it in order to raise or lower the temperature of a space. They are a cost effective and efficient way to both heat and cool your home. Learn more about air source heat pumps.

Air-to-air heat pump

Another name for an air source heat pump, see above.

Annual Fuel Efficiency Ratio (AFUE)

Similar to SEER (Seasonal Energy Efficiency Ratio), AFUE measures the energy efficiency of your heating system like a furnace or boiler. It tells you how much of your fuel is used to heat your home, and how much energy is lost through the process. For example, a furnace with a 80% AFUE rating means that 80% of the fuel is converted into heat while 20% is lost through exhaust. AFUE and SEER ratings go down over time. If your AFUE rating is below 80%, consider replacing your unit with a more energy-efficient one, like an air source heat pump.


British Thermal Unit (BTU)

A BTU is a measurement of energy, or heat, that is used to indicate the rate of cooling, dehumidifying, or heating in an HVAC system. One BTU is the amount of energy needed to raise the temperature of one pound of water by one degree Fahrenheit.


Carbon footprint

A carbon footprint is the total amount of greenhouse gas emissions created by an individual, organization, event, service, or product – such as a heating system – expressed by carbon dioxide equivalent. You can learn more about what your carbon footprint might be with our guide.


A chiller removes heat from a liquid through a vapor-compression or absorption refrigeration cycle. After it’s cooled, the liquid passes through the air conditioning unit (like a heat pump) and cools and dehumidifies the air.

Closed loop

A closed loop heat pump is a variety of geothermal heat pump that passes a mixture of water and antifreeze through a closed loop of pipe buried in the ground to collect heat.


A compressor, also referred to as a condenser or outdoor unit, is part of an ASHP or HVAC unit that is located outside. It compresses and pumps refrigerant to meet cooling needs, and contains the condenser coil

Condenser coil

A condenser coil is a component of air conditioning units that is involved in the basic refrigeration cycle that adds or removes heat from the system. The condenser is the hot side of an air conditioner or heat pump.

Cubic feet per minute (CFM)

Cubic feet per minute is a unit of measurement that determines airflow volume based on how many cubic feet of air pass by a non-moving point in one minute.

Cold-climate heat pump 

A cold-climate heat pump, also known as a low ambient heat pump, uses an inverter, or variable speed drive, that makes it capable of efficiently heating homes in colder climates with temperatures that get down to approximately -25 degrees Celsius or -13 degrees Fahrenheit. Previously, heat pumps were typically most effective in milder climates. However, advances in refrigeration technology have allowed manufacturers to create heat pumps that can be used in much colder climates.



A damper is a device that opens and closes to regulate how much air flows through vents, or ducts in a ducted heat pump system.


Equipment that reduces the level of humidity from the air. It works by cooling air to the point where water turns to liquid from vapor form, which is then removed.


A diffuser is placed over HVAC/ASHP ductwork. The diffuser separates air and distributes evenly in different directions. 

Direct expansion heat pump 

A direct expansion heat pump is a type of geothermal ground-source heat pump in which refrigerant circulates through a pipe buried in the ground to collect thermal energy.

Drain pan heater

A drain pan heater is an add-on unit for air source heat pumps that consists of a heating element. It can be added to the outdoor condenser unit of a heat pump in a cold climate to warm the drain pan, so ice does not form in the drain pan or at the base of the condenser unit.

Ducted heat pump

Ducted heat pumps, also known as central or forced air heat pumps, are connected to ductwork inside walls and ceilings and use those ducts to move warm or cool air throughout a building.

Ductless heat pump

See mini splits below.


Ductwork consists of specialized pipes or channels that direct airflow (including supply air, return air and exhaust air) within a home or building.


Efficiency rating

An efficiency rating is a ratio that measures the efficiency of a heat pump (or any other device). Annual heating efficiency is measured by HSPF, while annual cooling efficiency is measured by SEER.


ENERGY STAR is a program run by the U.S. Department of Energy (DOE) and the U.S. Environmental Protection Agency (EPA) to promote energy efficiency. You may recognize the ENERGY STAR logo from appliances you have purchased previously. It certifies that the appliance or heating system has met certain standards of energy efficiency set by the DOE and EPA.

Evaporator coil

An evaporator coil is part of the refrigeration cycle that absorbs or adds heat to the system.


Fresh air intake

Fresh air intake is an opening through which outside air is drawn into your home to replace any air being removed by the ventilation and circulation systems.


Geothermal heat pumps

Geothermal heat pumps, also referred to as ground source, water source, or earth-coupled heat pumps, are a type of heat pump that collects thermal energy from the ground or a water source and transfers that energy inside a building to heat an indoor space.


Heating coil

A heating coil is a part of the HVAC system that conducts heat within the appliance.

Heat pump

A heat pump is a compressor that cycles both hot and cold air. It can heat or cool a room or your entire home (depending on the system) by absorbing heat from a cold space and transferring it to a warmer space.

Heat output

Heat output is the amount of thermal energy a heat pump releases into to warm a space. It is measured in BTUs.

Heat transfer

Heat transfer occurs when heat is moved from one area to another, heating or cooling an indoor area.


HVAC is the acronym for heating, ventilation and air conditioning systems. More modern HVAC systems will also include air cleaning and moisture control features. 


Kilowatt (kW)

A kilowatt is a measure of power equal to 1,000 Watts. A Watt is a unit used to quantify the rate of energy transfer. You will most commonly see kW as the measurement when it comes to air source heat pumps and HVAC.

Kilowatt-hour (kWh)

A kilowatt-hour unit that measures the amount of electricity used in an hour, equal to one kilowatt of power sustained for one hour, or 3,600 kilojoules.



A line-set is a pair of copper tubes that connect a condenser to an evaporator so refrigerant can move between the two. The smaller tube is called a liquid or discharge line and carries the liquid refrigerant to the evaporator. The larger tube is called a suction line, and it moves refrigerant in its gaseous form back to the condenser.

Load calculation

Load calculation determines the size of heat pump necessary to provide proper temperature control in a desired area (a single room, a floor, or your entire home). Load calculations analyze factors including air volume of the desired area and the level of insulation in your home.

Low ambient

See cold climate heat pumps.


Manual J

Manual J is a calculation that determines the size that an HVAC unit needs to be to properly heat and cool a building as efficiently as possible. It is measured in BTUs and it is the only procedure recognized by the American National Standards Institute (ANSI) and required by residential building codes. A Manual J is also frequently necessary for you to be eligible for any state and local incentives.

Mini split

A mini split is a ductless heat pump system in which an outdoor compressor unit is combined with multiple air handlers inside, usually one in every high-use room. It allows you to control the thermostat/temperature in different areas of the home. 


Open loop

An open loop is a geothermal heat pump that collects thermal energy from groundwater or water from another nearby source. It takes water from its source, moves it through a loop of pipe to collect the thermal energy, and transfers it back into the source.

Outside air temperature (OAT)

Outside air temperature is the air temperature outside a building. The outdoor temperature range where you live determines the type of heat pump you will need, as some operate better than others in colder climates.

Outdoor unit

See compressor.


Packaged unit

A packaged unit is an air-handling unit, defined as either “recirculating” or “once-through” design, created for outdoor installation. They most often include, internally, their own heating and cooling devices.



Refrigerant is a fluid used in both heat pumps and air conditioning systems that can change from a gas to a liquid and back again repeatedly. It allows the heat pump to regulate the air temperature inside your home by producing a cooling effect as it changes from a liquid to a gas.

Refrigeration cycle

The refrigeration cycle transfers thermal energy from a colder space to a warmer space. This cycle is a reverse thermodynamic cycle that performs the opposite of the type of energy transfer that happens naturally without intervention from a system such as a heat pump or HVAC unit.


Seasonal Energy Efficiency Ratio (SEER)

The seasonal energy efficiency ratio measures the energy efficiency of your air conditioning system. It is equal to the Total Cooling Output Over the Cooling Season / Total Electrical Energy Input Over the Cooling Season. It measures the total cooling of your air conditioner or heat pump in BTUs compared with the energy output (in watt hours) used within the same period. The higher the SEER rating on your air conditioner, the more energy efficient it is, meaning lower energy costs to run it.

Single-zone heat pump

A single-zone heat pump is a single compressor connected to a single air handler, instead of one outdoor unit connected to multiple indoor air handlers.

Split system (zoned)

See mini split.



A thermostat monitors and regulates a heating or cooling system. It can be used to set the desired temperature at which it keeps the environment either warm or cool.


Variable air volume (VAV) system

A variable air volume system is an HVAC system that provides a stable supply-air temperature, and varies the airflow rate to meet the temperature requirements. They conserve energy by operating at lower fan speeds during times of lower temperature control demand.



A Watt is a unit of power that is used to quantify the rate of energy transfer, and electricity is measured in Watts. It is equal to one joule of work per second. In these articles, we break down how many Watts air source heat pumps and air conditioners use.

Power your HVAC system with solar

Looking to install an ASHP or central AC? By powering your HVAC system with solar energy, you’ll be able to significantly reduce your electricity bills (while helping the environment!). Check out the EnergySage Marketplace to receive multiple quotes from local installers. Comparing quotes will allow you to find a system that meets your needs and will power your HVAC system at the right price!